1. Field of the Invention
The present invention relates to a thin film transistor device having thin film transistors (abbreviated as “TFTs” hereinafter) used in a liquid crystal display panel, an organic EL display panel, etc. and a method of manufacturing the same.
2. Description of the Prior Art
The liquid crystal display panel has such merits that such panel is thin and light in weight and the consumption power is small because such panel can be driven by the low voltage, and is widely employed in various electronic devices such as PDA (Personal Digital Assistant), the finder of the video camera, and others. In particular, since the active-matrix liquid crystal display panel, in which the switching element such as TFT or the like is provided to each pixel, is excellent in display quality to such an extent that it is equivalent to the CRT (Cathode Ray Tube), such display panel is employed in the display device of the television set, the personal computer, etc.
The normal TN (Twisted Nematic) liquid crystal display panel has the structure that the liquid crystal is sealed between two sheets of transparent glass substrates. Out of two mutually-opposing surfaces (opposing surfaces) of these glass substrates, the black matrix, the color filters, the common electrode, etc. are formed on one surface side, and the TFTs, the pixel electrodes, etc. are formed on the other surface side.
Also, the polarization plate is provided to the opposing surfaces and opposite surfaces of the glass substrates respectively. These two sheets of polarization plates are arranged such that the polarization axes of the polarization plates are set to intersect orthogonally with each other, for example. According to this state, the display panel is set to the mode in which the light is transmitted when the electric field is not applied and the light is shielded when the electric field is applied, i.e., the normally white mode. Also, if the polarization axes of two sheets of polarization plates are set in parallel with each other, the display panel is set to the mode in which the light is shielded when the electric field is not applied and the light is transmitted when the electric field is applied, i.e., the normally black mode. In the following explanation, the substrate on which the TFTs and the pixel electrodes are formed is called the TFT substrate and the substrate on which the color filters and the common electrode are formed is called the CF substrate.
At present, the operating layer of TFT that is formed of amorphous silicon is employed in the normal liquid crystal display panel. However, since the carrier mobility is small in the amorphous silicon, there exists such a drawback that it is hard to apply such operating layer to the large and high-definition liquid crystal display panel. For this reason, the operating layer of TFT that is formed of polysilicon has been proposed, and such operating layer has already been used practically in a part of products.
In such liquid crystal display panel, not only the TFTs formed in the pixel portion (referred to as “pixel TFTs” hereinafter) but also the peripheral circuits such as the data driver, the gate driver, etc., which are constructed by CMOS, etc., can be formed integrally on the substrate. Thus, there is the merit such that the production cost of the liquid crystal display panel can be considerably reduced. In this case, since the OFF current is large in the polysilicon TFT, the LDD (Lightly Doped Drain) structure must be employed in the pixel TFTs.
FIGS. 1A to 1L are sectional views showing a method of manufacturing a TFT substrate of the liquid crystal display panel in the prior art in order of step. In these Figures, for convenience of explanation, the pixel TFT (n-type TFT) is illustrated on the left side and also the p-type TFT of the peripheral circuit is illustrated on the right side. Actually, the pixel TFTs are formed in the display region and the peripheral circuit is formed on the outside of the display region. Also, since the n-type TFTs of the peripheral circuit can be formed similarly to the pixel TFTs, illustration and explanation of them will be omitted herein.
First, as shown in FIG. 1A, a buffer layer 12 having a double-layered structure consisting of a SiN film 12a and a SiO2 film 12b is formed on a glass substrate 11. Then, an amorphous silicon film is formed on the SiO2 film 12b, and then the amorphous silicon is changed into polysilicon by the annealing using the excimer laser to form a polysilicon film 13. Then, photoresist is coated on the polysilicon film 13, and then photoresist films 14 each having a predetermined shape are formed by applying the exposing and developing processes to the photoresist.
Then, as shown in FIG. 1B, the polysilicon film 13 is etched by using the photoresist films 14 as a mask and thus the polysilicon film 13 is left only in the TFT forming region. Then, the photoresist films 14 are removed.
Then, as shown in FIG. 1C, an insulating film 15 and a conductive film 16 are formed sequentially on an overall upper surface of the glass substrate 11. Then, as shown in FIG. 1D, gate electrodes 19 and gate insulating films 18 are formed by etching the conductive film 16 and the insulating film 15 by virtue of the photolithography method. At this time, a width of the gate electrode 19 is formed slightly narrowly rather than a width of the gate insulating film 18.
Then, as shown in FIG. 1E, source/drain regions of the n-type TFT are formed by ion-implanting p (phosphorus) into the polysilicon film 13. For example, high-concentration impurity diffusion regions 13b are formed by ion-implanting P into the polysilicon film 13 with a high concentration at a low energy while using the gate insulating film 18 and the gate electrode 19 as a mask. Also, LDD regions 13a of the n-type TFT are formed by ion-implanting P into the polysilicon film 13 with a low concentration at a high energy while using the gate electrode 19 as a mask.
Then, as shown in FIG. 1F, a resist film 20 for covering the n-type TFT is formed. Then, source/drain regions of the p-type TFT are formed by ion-implanting B (boron) into the polysilicon film 13 in the p-type TFT forming region. For example, high-concentration impurity diffusion regions 13d are formed by ion-implanting B into the polysilicon film 13 with a high concentration at a low energy while using the gate insulating film 18 and the gate electrode 19 as a mask. Also, LDD regions 13c of the p-type TFT are formed by ion-implanting B into the polysilicon film 13 with a low concentration at a high energy while using the gate electrode 19 as a mask. In this manner, the n-type TFT can be changed into the p-type TFT by implanting the p-type impurity into the polysilicon film 13, into which the n-type impurity has been implanted, in larger quantity than the n-type impurity. Then, the resist film 20 is removed. 
Then, as shown in FIG. 1G, the impurity being introduced into the polysilicon film 13 is activated by irradiating the excimer laser onto the polysilicon film 13.
Then, as shown in FIG. 1H, a SiO2 film 21 and a SiN film 22 are formed sequentially as an interlayer insulating film on an overall upper surface of the glass substrate 11. Also, an ITO (Indium-Tin Oxide) film 23 is formed on the SiN film 22 and also a resist film 24 is formed on the ITO film 23 in the pixel-electrode forming region.
Then, as shown in FIG. 1I, a pixel electrode 25 is formed by etching the ITO film 23 using the resist film 24 as a mask. Then, the resist film 24 on the pixel electrode 25 is removed.
Then, as shown in FIG. 1J, photoresist is coated on an overall upper surface of the glass substrate 11, and then a resist film 26 in which respective portions corresponding contact-hole forming regions are opened is formed by applying the exposing and developing processes to the photoresist. Then, as shown in FIG. 1K, contact holes 22a reaching the high-concentration impurity diffusion regions 13b, 13d from the surface of the SiN film 22 are formed by etching the SiN film 22 and the SiO2 film 21 while using the resist film 26 as a mask. Then, the resist film 26 is removed.
Then, as shown in FIG. 1L, a metal film is formed on an overall upper surface of the glass substrate 11, and then predetermined wirings (containing the source/drain electrodes) 27 are formed by patterning the metal film by means of the photolithography method. The source region of the pixel TFT is connected electrically to the pixel electrode 25 via the wiring 27. Also, the drain region of the pixel TFT is connected to the data bus line via other wiring 27. In this manner, the TFT substrate of the liquid crystal display panel is completed.
However, the inventors of the present invention consider that the above TFT substrate manufacturing method in the prior art contains the problem described in the following. That is, the method in the prior art needs a number of masking steps, which cause the increase of the production cost. The sub-steps such as the photoresist coating step, the pre-baking step, the exposing step, the developing step, the post-baking step, etc. are contained in the masking steps. Therefore, if the number of the masking steps can be reduced, the production cost of the product can be considerably reduced.